Method of driving a light emitting device

ABSTRACT

The present invention is characterized in that a transistor with its L/W set to 10 or larger is employed, and that |V DS | of the transistor is set equal to or larger than 1 V and equal to or less than |V GS −V th |. The transistor is used as a resistor so that the resistance of a light emitting element can be held by the transistor. This slows down an increase in internal resistance of the light emitting element and the resultant current value reduction. Accordingly, a change with time in light emission luminance is reduced and the reliability is improved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/955,154, filed Jul. 31, 2013, now allowed, which is a continuation ofU.S. application Ser. No. 13/347,780, filed Jan. 11, 2012, now U.S. Pat.No. 8,502,241, which is a continuation of U.S. application Ser. No.12/238,035, filed Sep. 25, 2008, now U.S. Pat. No. 8,101,439, which is acontinuation of U.S. application Ser. No. 10/425,708, filed Apr. 30,2003, now U.S. Pat. No. 7,445,946, which claims the benefit of a foreignpriority application filed in Japan as Serial No. 2002-129424 on Apr.30, 2002, all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique of a light emitting deviceusing a light emitting element, and more specifically to a technique ofa light emitting device controlling an applied voltage of the lightemitting element with electric field effect type transistors.

2. Description of the Related Art

In recent years, the development of a display device for displaying animage has been progressed. As the display device, a liquid crystaldisplay device for displaying an image using a liquid crystal elementhas been widely used for a display screen of a mobile telephone bytaking advantages of a high image quality, a thin type, a light weight,and the like.

On the other hand, in recent years, the development of a light emittingdevice using a light emitting element has been also progressed. Thelight emitting device has features such as a high response speed,superior moving picture display, and wide viewing characteristic inaddition to advantages of the existing liquid crystal display device.Thus, it has been noted as a next-generation compact mobile flat paneldisplay capable of using moving picture contents.

The light emitting element is made of broad materials such as an organicmaterial, an inorganic material, a thin film material, a bulk material,or a dispersion material. Of them, as a typical light emitting element,there is an organic light emitting diode (OLED) mainly made of anorganic material. The light emitting element has a structure in which ananode, a cathode, and a light emitting layer sandwiched between theanode and the cathode are provided. The light emitting layer is made ofone or plural materials selected from the above-mentioned materials.Note that the amount of current flowing between both electrodes of thelight emitting element is in direct proportion to light emissionluminance.

In many cases, a plurality of pixels each having a light emittingelement and at least two transistors are provided in the light emittingdevice. In each of the pixels, a transistor connected in series with thelight emitting element (hereinafter indicated as a driver transistor)has a function for controlling light emission of the light emittingelement. When a gate-source voltage (hereinafter indicated as V_(GS)) ofa driver transistor and a source-drain voltage (hereinafter indicated asV_(DS)) thereof are changed as appropriate, the driver transistor cartbe operated in a saturation region or a nonsaturation region.

When the driver transistor is operated in the saturation region(|V_(GS)−V_(th)|<|V_(DS)|), the amount of current flowing between bothelectrodes of the light emitting element is greatly dependent on achange in |V_(GS)| of the driver transistor but not dependent on achange in |V_(DS)|. Note that a drive method of operating the drivertransistor in the saturation region is called constant current drive.FIG. 9A is a schematic view of a pixel to which the constant currentdrive is applied. In the constant current drive, a gate electrode of thedriver transistor is controlled to flow the necessary amount of currentinto the light emitting element. In other words, the driver transistoris used as a voltage control current source and the driver transistor isset such that a constant current flows between a power source line andthe light emitting element.

On the other hand, when the driver transistor is operated in thenonsaturation region (|V_(GS)−V_(th)|>|V_(DS)|), the amount of currentflowing between both electrodes of the light emitting element is changedaccording to both values of |V_(GS)| and |V_(DS)|, more specifically,|V_(DS)| is changed depending on the value of |V_(GS)| and in a rangewithin 1V at the maximum. Note that a drive method of operating thedriver transistor in non-saturation region is called constant voltagedrive. FIG. 9B is a schematic view of a pixel to which the constantvoltage drive is applied. In the constant voltage drive, the drivertransistor is used as a switch, and a power source line and the lightemitting element are shorted if necessary, thereby flowing a currentinto the light emitting element.

There is provided a light emitting device capable of displaying clearmulti-gradation colors using pixels which perform such constant voltagedriving. Further, there also is provided a light emitting deviceapplicable to a time-gradation method. (see Patent References 1 and 2).

[Patent Reference 1] JP 2001-343933 A

[Patent Reference 2] JP 2001-5426 A

Light emitting elements by nature increase their resistance (internalresistance) with time. An increase in internal resistance causesreduction in amount of current flowing between anodes and cathodes oflight emitting elements because the current is in reverse proportion tothe resistance. In short, the luminance of a light emitting element islowered with time and this makes it difficult to obtain a desired lightemission luminance.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above, and an objectof the present invention is therefore to provide a method of driving alight emitting device by constant voltage driving which can slow downcurrent reduction with time and improve the reliability.

According to an aspect of the present invention, there is provided amethod of driving a light emitting device having a light emittingelement and a driving transistor, the transistor being connected to thelight emitting element, the transistor having a channel with W and achannel length L which satisfy L/W≧10,

the method being characterized in that a voltage is applied to a gateelectrode of the driving transistor and to a drain electrode or sourceelectrode thereof so that a gate-source voltage V_(GS), source-drainvoltage V_(DS), and threshold voltage V_(th) of the driving transistorsatisfy 1≦|V_(DS)|≦|V_(GS)−V_(th)|.

According to another aspect of the present invention, there is provideda method of driving a light emitting device having a light emittingelement and first and second driving transistors, the transistors beingconnected to the light emitting element, the transistors having achannel width W and a channel length L which satisfy L/W≧10,

the method being characterized in that:

the first and second driving transistors are connected in series;

a channel width W₁ and channel length L₁ of the first driving transistorand a channel width W₂ and channel length L₂ of the second drivingtransistor satisfy (L₁+L₂)/W₁≧10 and (L₁+L₂)/W₂≧10;

a voltage is applied to a gate electrode of the first driving transistorand to a drain electrode or source electrode thereof so that agate-source voltage V_(GS1), source-drain voltage V_(DS1), and thresholdvoltage V_(th1) of the first driving transistor satisfy1≦|V_(DS1)|≦|V_(GS1)−V_(th1)|; and

a voltage is applied to a gate electrode of the second drivingtransistor and to a drain electrode or source electrode thereof so thata gate-source voltage V_(GS2), source-drain voltage V_(DS2), andthreshold voltage V_(th2) of the second driving transistor satisfy1≦|V_(DS2)|≦|V_(GS2)−V_(th2)|.

To summarize the above, a light emitting device to which the presentinvention is applied is characterized by using a driving transistor withits L/W set to equal to or larger than 10. The present invention ischaracterized by operating a driving transistor with its |V_(DS)|, whichis nearly zero in prior art, set equal to or larger than 1 V and equalto or smaller than |V_(GS)−V_(th)|. Operating the driving transistor at|V_(DS)| in the above range makes it possible to use the drivingtransistor as a resistor. This makes the current flowing betweenelectrodes of a light emitting element inversely related to the sum ofresistance of the light emitting element and resistance of the drivingtransistor. In short, the current value in the present invention is inreverse proportion to the sum of resistance values of the light emittingelement and of the driving transistor whereas the current value in priorart is in reverse proportion to the resistance value of the lightemitting element alone. As a result, the reduction with time in currentvalue of the light emitting element can be slowed down. Accordingly,lowering of light emission luminance with time is reduced and thereliability is improved.

The present invention is also characterized in that a voltage is appliedto a gate electrode of the driving transistor and to a drain electrodeor a source electrode thereof so that the driving transistor operateswith its |V_(DS)| set equal to or larger than 1 V and equal to orsmaller than |V_(GS)−V_(th)|. In other words, the present invention ischaracterized in that a voltage is applied to a signal line forinputting a signal to the gate electrode of the driving transistor andis applied to a power supply line connected with the source electrode ordrain electrode of the driving transistor to give these linesappropriate electric potentials. To elaborate, the invention ischaracterized in that the electric potential of a signal to be inputtedto the gate electrode of the driving transistor, the electric potentialof a signal line driving circuit connected to a signal line that outputsthe above signal, the electric potential of a power supply lineconnected to the source electrode or drain electrode of the drivingtransistor, and the electric potential of a power supply circuitconnected to the power supply line are set to their respectiveappropriate levels.

The present invention employs a transistor with its L/W set to 10 orlarger and therefore is characterized in that |V_(GS)| of the drivingtransistor is held by a capacitor between the gate electrode and channelformation region of the driving transistor. In other words, a transistorin the present invention can double as a capacitor element and influenceof fluctuation in characteristic of the transistor itself is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are diagrams illustrating voltage-currentcharacteristics;

FIGS. 2A to 2E are diagrams illustrating effects of the presentinvention;

FIGS. 3A and 3B are diagrams showing simulation results;

FIGS. 4A to 4D are diagrams showing a light emitting device according tothe present invention;

FIGS. 5A and 5B are diagrams showing a light emitting device accordingto the present invention;

FIGS. 6A to 6H are diagrams showing electronic apparatuses to which thepresent invention is applied;

FIGS. 7A and 7B are diagrams showing layout of pixels in accordance withthe present invention;

FIGS. 8A and 8B are diagrams showing layout of pixels in accordance withthe present invention;

FIGS. 9A and 9B are conceptual diagrams of constant current driving andconstant voltage driving, respectively; and

FIGS. 10A to 10C are diagrams illustrating voltage-currentcharacteristics.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment Mode 1

An embodiment mode of the present invention will be described withreference to FIGS. 1A to 4D.

FIG. 4A shows an outline of a light emitting device to which the presentinvention is applied. The light emitting device has a pixel portion 302and a signal line driving circuit 303 and scanning line driving circuit304 which are placed in the periphery of the pixel portion 302.

The pixel portion 302 has x signal lines S₁ to S_(x) and x power supplylines V₁ to V_(x) which are arranged in the column direction, as well asy scanning lines G₁ to G_(y) and y power supply lines C₁ to C_(y) whichare arranged in the row direction (x and y are natural numbers). Aregion surrounded by one of the x signal lines S₁ to S_(x), one of the xpower supply lines V₁ to V_(x), one of the y scanning lines G₁ to G_(y),and one of the y power supply lines C₁ to C_(y) corresponds to a pixel301. In the pixel portion 302, the pixel 301 and similarly structuredpixels are arranged to form a matrix pattern.

The signal line driving circuit 303 and the scanning line drivingcircuit 304 may be formed integrally on the same substrate where thepixel portion 302 is formed. Alternatively, the driving circuits may beplaced outside of the substrate on which the pixel portion 302 isformed. The light emitting device may have more than one signal linedriving circuit 303 and more than one scanning line driving circuit 304.Arbitral numbers can be set for the signal line driving circuit 303 andthe scanning line driving circuit 304 so as to suit the structure of thepixel portion 301. Signals and power are supplied to the signal linedriving circuit 303 and the scanning line driving circuit 304 through anFPC or the like (not shown in the drawing) from the external. The powersupply lines C₁ to C_(y) are connected to a power supply circuit, whichmay be integrated with the pixel portion 302 or may be external andconnected to the pixel portion 302 through an FPC or the like.

Light emitting devices in the present invention include a light emittingpanel in which a pixel portion with a light emitting element and drivingcircuits are sealed between a substrate and a cover member, a lightemitting module obtained by mounting an IC or the like to the lightemitting panel, and a light emitting display for use as a displaydevice. In short, ‘light emitting device’ is used as the generic termfor a light emitting panel, a light emitting module, a light emittingdisplay, and the like.

The pixel 301 is on the i-th column and j-th row in the pixel portion302. Two typical structural examples for the pixel 301 will be given anddetails thereof are described with reference to FIGS. 4B and 4C. Thepixel 301 shown in FIG. 4B has a switching transistor 306, a drivingtransistor 307, and a light emitting element 308. The pixel 301 shown inFIG. 4C has an erasing transistor 309 and a scanning line R_(j) inaddition to the components of the pixel 301 of FIG. 4B.

In FIGS. 4B and 4C, the switching transistor 306 has a gate electrodewhich is connected to a scanning line G_(j). The switching transistor306 also has a first electrode connected to a signal line S_(i) and asecond electrode connected to a gate electrode of the driving transistor307. A first electrode of the driving transistor 307 is connected to apower supply line V_(i) and a second electrode thereof is connected toone of electrodes of the light emitting element 308. The other electrodeof the light emitting element 308 is connected to a power supply lineC_(j).

In FIG. 4C, the switching transistor 306 and the erasing transistor 309are connected to each other in series between a signal line S_(i) and apower supply line V_(i). The erasing transistor 309 has a gate electrodeconnected to the scanning line R_(j).

In this specification, one of the electrodes of the light emittingelement 308 that is connected to the second electrode of the drivingtransistor 307 is called a pixel electrode and the other electrodeconnected to the power supply line C_(j) is called an oppositeelectrode.

The switching transistor 306 in FIGS. 4B and 4C has a function ofcontrolling input of signals to the pixel 301. The switching transistor306 only has to have the function of a switch and therefore theconductivity type thereof is not particularly limited; the switchingtransistor 306 can be the n-channel type and the p-channel type both.

The driving transistor 307 in FIGS. 4B and 4C has a function ofcontrolling light emission of the light emitting element 308. Theconductivity type of the driving transistor 307 is not particularlylimited. When the driving transistor 307 is a p-channel transistor, thepixel electrode serves as an anode whereas the opposite electrode servesas a cathode. When the driving transistor 307 is an n-channeltransistor, the pixel electrode serves as a cathode whereas the oppositeelectrode serves as an anode.

The erasing transistor 309 in FIG. 4C has a function of stopping lightemission of the light emitting element 308. The erasing transistor 309only has to have the function of a switch and therefore the conductivitytype thereof is not particularly limited; the erasing transistor 309 canbe an n-channel type transistor and a p-channel type transistor.

The transistors placed in the pixel 301 can have a single-gate structurewith one gate electrode as well as a multi-gate structure such as adouble-gate structure with two gate electrodes and a triple-gatestructure with three gate electrodes. Also, the transistors may have atop gate structure in which a gate electrode is placed above asemiconductor or a bottom gate structure in which a gate electrode isplaced below a semiconductor.

The light emitting device to which the present invention is applied ischaracterized in that a channel length L of the driving transistor 307is set long. Specifically, the device is characterized in that thechannel length L is set several to several hundred times longer than achannel width W of the driving transistor 307.

Referring to FIGS. 1A and 1B and FIGS. 10A to 10C, a description isgiven on the voltage-current characteristic of a standard-sizedtransistor which is designed to have a standard (common) L/W value, 0.5,and the voltage-current characteristic of a long-sized transistor of thepresent invention in which L/W is 100. Also described is thevoltage-current characteristic of the light emitting element 308 whenthe driving transistor 307 is the long-sized transistor.

FIG. 1A shows a portion of the pixel 301 of FIGS. 4A to 4D where thedriving transistor 307 is connected to the light emitting element 308.In FIG. 1A, the source electrode of the driving transistor 307 which isconnected to the power supply line V_(i) is denoted by 101 and the gateelectrode of the driving transistor 307 is denoted by 102. The pixelelectrode of the light emitting element 308 (the drain electrode of thedriving transistor 307) is denoted by 103 and the opposite electrode isdenoted by 104. The source-drain voltage of the driving transistor 307is denoted by V_(DS). The voltage between the pixel electrode 103 andthe opposite electrode 104 is denoted by V_(EL).

FIG. 1B shows voltage-current characteristics 106 and 107 of whenV_(GS1) and V_(GS2) (V_(GS1)<V_(GS2)) are applied to the long-sizedtransistor, voltage current characteristics 108 and 109 of when V_(GS3)and V_(GS4) (V_(GS3)<V_(GS4)) are applied to the standard-sizedtransistor, and a voltage-current characteristic 110 of the lightemitting element 308. Since the driving transistor 307 and the lightemitting element 308 are connected in series, the same amount of currentflows in the transistor and the light emitting element. Accordingly, thedriving transistor 307 and the light emitting element 308 are driven atan intersection point (operation point) of the curves that indicatetheir voltage-current characteristics.

As shown in FIG. 1B, according to the voltage-current characteristics ofthe standard-sized and long-sized transistors, a current value I_(D)rises as V_(DS) is increased. Then the current value I_(D) reachessaturation when V_(DS) passes a certain voltage level. The V_(DS) valueat which the current value I_(D) reaches saturation varies depending onV_(GS).

Here, the desired value of current flowing between the electrodes of thelight emitting element 308 is given as I_(D107) and a portion 111 ofFIG. 1B is enlarged in FIG. 10A. FIG. 10B shows the voltage-currentcharacteristics 108 and 109 of the standard-sized transistor and thevoltage-current characteristic 110 of the light emitting element 308.FIG. 10C shows the voltage-current characteristics 106 and 107 of thelong-sized transistor and the voltage-current characteristic 110 of thelight emitting element 308.

As shown in FIG. 10A, in the region denoted by 111, the curves 108 and109 of the standard-sized transistor are sharply inclined whereas thecurves 106 and 107 of the long-sized transistor are gently inclined.

The difference in inclination originates from the difference in L/Wbetween the transistors. The standard-sized transistor has an L/W valueof 0.1 to 2 and therefore V_(DS) of the transistor cannot be set large.The V_(DS) value is far smaller than V_(EL) and nearly zero as shown inFIG. 10B. In short, the curve of the standard-sized transistor has asharp inclination because the drain current I_(D) is rapidly increasedaccompanying a change in V_(DS) and reaches saturation when V_(DS)passes a certain voltage level.

In contrast, the long-sized driving transistor is characterized bysetting L/W to 10 or larger and operating with its |V_(DS)| set equal toor larger than 1 V and equal to or smaller than |V_(GS)−V_(th)|.Operating the driving transistor at the above range of |V_(DS)| makes itpossible to use the driving transistor as a resistor. Therefore, asshown in FIG. 10C, the difference between V_(DS) and V_(EL) of thelong-sized transistor is not large and a change in V_(DS) causes a slowincrease in drain current I_(D). Its voltage-current characteristiccurve accordingly has a gentle inclination.

The present invention employs the driving transistor 307 with its L/Wset to 10 or larger and appropriate voltages are applied to the powersupply line V_(i) connected with the driving transistor 307 and to thegate electrode to set the electric potentials of the power supply lineV_(i) and the gate electrode to appropriate levels. In this way,|V_(DS)| of the driving transistor 307 which is less than 1V in priorart can be set equal to or larger than 1 V and equal to or smaller than|V_(GS)−V_(th)|. Since the resistance of a transistor also depends on|V_(DS)| of the transistor, setting |V_(DS)| equal to or larger than 1 Vand equal to or smaller than |V_(GS)−V_(th)| causes resistance (internalresistance) in the driving transistor. As a result, the current flowingbetween the electrodes of the light emitting element becomes inverselyrelated to the sum of resistance of the light emitting element and ofthe transistor. In short, the current value in the present invention isin reverse proportion to the sum of resistance of the light emittingelement and of the driving transistor whereas the current value in priorart is in reverse proportion to the resistance of the light emittingelement alone. The current value reduction with time of the lightemitting element thus can be slowed down. A detailed description isgiven on this effect with reference to FIGS. 2A to 2E.

Within the above |V_(DS)| range, the resistance of the transistor canslow down the reduction of the current value. In other words, theresistance of the transistor when |V_(DS)| is less than 1 V is too smallto hold back the reduction in current value whereas the transistoroperates in a saturation range when |V_(DS)| is larger than|V_(GS)−V_(th)|.

FIGS. 2A and 2B show a portion where the long-sized driving transistor307 and the light emitting element 308 are connected to each other.FIGS. 2C and 2D show a portion where the standard-sized drivingtransistor 307 and the light emitting element 308 are connected to eachother. The resistance of the transistor 307 is given as R_(T) and theresistance of the light emitting element 308 is given as R_(E).

In FIGS. 2A and 2B, when one of the electrodes of the light emittingelement 308 is grounded, the current value I_(DL) satisfies thefollowing Expression (1).

I _(DL) =V _(DDL)/(R _(T) +R _(E))  (1)

In Expression (1), the resistance R_(T) of the transistor 307 and theresistance R_(E) of the light emitting element 308 are substantiallyequal to each other. With time, the resistance R_(T) of the transistor307 is reduced and the resistance R_(E) of the light emitting element308 is increased. Then the current value I_(DL) of the current flowingin the light emitting element 308 satisfies the following Expression(2).

I _(DL) =V _(DDL)/(R _(T) ′+R _(E)′)  (2)

Assuming that the resistance of the light emitting element 308 becomeswith time R_(E)′ which satisfies R_(E)′≈2×R_(E), the rate of change ofthe current value I_(DL) is ⅓. More accurately, R_(T) is larger thanR_(T)′ (R_(T)>R_(T)′) in the above Expressions (1) and (2) since theresistance R_(E) of the light emitting element 308 increases and theresistance R_(T) of the transistor 307 declines with time. The exactrate of change of the current value I_(DL) is therefore less than ⅓.

Similarly, in FIGS. 2C and 2D, when one of the electrodes of the lightemitting element 308 is grounded, a current value I_(DS) of the currentflowing in the light emitting element 308 satisfies the followingExpression (3). The standard-sized transistor 307 has substantially noresistance R_(T) and therefore the resistance R_(T) here is assumed aszero.

I _(DS) =V _(DDS) /R _(E)  (3)

As the resistance of the light emitting element 308 is increased withtime, the current value I_(DS) of the current flowing in the lightemitting element 308 now satisfies the following Expression (4).

I _(DS) =V _(DDS) /R _(E)′  (4)

Assuming that the resistance of the light emitting element 308 becomesR_(E)′ which satisfies R_(E)′=2×R_(E), the rate of change of the currentvalue I_(DS) is ½.

To summarize the above, if the resistance of the light emitting element308 satisfies R_(E)′=2×R_(E), the rate of change of the current value is½ when the transistor used is the standard-sized transistor whereas itis about ⅓ when the transistor used is the long-sized transistor. Theuse of the long-sized transistor thus reduces apparent rate of change.

As described, the current value is in reverse proportion to theresistance of the light emitting element alone when the standard-sizedtransistor is used. On the other hand, when the long-sized transistor ofthe present invention in which L/W is set to 10 or larger is employed, avoltage is applied to the power supply line (not shown in the drawings)connected with the source electrode of the driving transistor to set thepower supply electric potential V_(DDL) of the power supply line to anappropriate level and a voltage is applied also to the gate electrode ofthe driving transistor. In this way, |V_(DS)| of the driving transistor307 which is less than 1V in prior art can be set equal to or largerthan 1 V and equal to or smaller than |V_(GS)−V_(th)|. Setting |V_(DS)|equal to or larger than 1 V and equal to or smaller than |V_(GS)−V_(th)|causes resistance (internal resistance) in the transistor. This makesthe value of the current flowing between the electrodes of the lightemitting element inversely related to the sum of resistance of the lightemitting element and of the transistor as shown in the equivalentcircuit of FIG. 2E. As a result, the current value reduction with timeof the light emitting element can be slowed down.

The present invention employs a transistor with its L/W set to 10 orlarger and therefore is characterized in that |V_(GS)| of the drivingtransistor is held by a capacitor between the gate electrode and channelformation region of the driving transistor. In other words, a transistorin the present invention can double as a capacitor element and influenceof fluctuation in characteristic of the transistor itself is reduced.

The present invention can be carried out by merely designing a drivingtransistor to have an L/W which is larger than usual and there is noneed to add another manufacturing step. Therefore the present inventioncan slow down the reduction of the current value without lowering theyield in the manufacturing process.

The resistance (internal resistance) of the light emitting element 308is changed not only with time but also by temperature shift because ofits nature. To elaborate, the resistance of the light emitting element308 declines when the temperature becomes higher than the now altemperature, namely room temperature, and rises when the temperaturebecomes lower than normal. According to FIG. 10B, V_(DS) of thestandard-sized transistor is far smaller than V_(EL) and is nearly zero.This means that, when the standard-sized transistor is used, the valueof the current flowing in the light emitting element is determinedmainly by the resistance of the light emitting element. If thetemperature rises higher than room temperature to cause a drop inresistance of the light emitting element, the light emission luminancesurges resulting in degradation of the light emitting element andburn-in of a display pattern. On the other hand, according to FIG. 10C,the curve of the long-sized transistor has a gentle inclination, thereis no large difference between V_(DS) and V_(EL), and the current valueis gradually increased accompanying a change in V_(DS). In short, whenthe long-sized transistor of the present invention is used, a change inresistance due to temperature shift does not cause a light emissionsurge.

Embodiment Mode 2

This embodiment describes simulation results of the standard-sized andlong-sized transistors with reference to FIGS. 3A and 3B. Table 1 belowshows for each transistor in the simulation the channel length L, thechannel width W, the absolute value |V_(th)| of the threshold, theabsolute value |V_(GS)| of the gate-source voltage, the electricpotential 101 of the drain electrode, the electric potential 102 of thegate electrode, and the electric potential 103 of the source electrode(the electric potential of the pixel electrode). Table 1 also shows theelectric potential 104 of the opposite electrode, and the current valueI and resistance R of the light emitting element 308.

Table 1

In FIG. 3A, the voltage-current characteristic of the long-sizedtransistor is denoted by 201 and the voltage-current characteristic ofthe standard-sized transistor is indicated by 202. As shown in FIG. 3A,the voltage-current characteristic 201 of the long-sized transistor hasa gentle inclination whereas the voltage-current characteristic 202 ofthe standard-sized transistor is sharply inclined. Both curves reachsaturation when a certain voltage level or over is attained.

A region denoted by 203 in FIG. 3A is enlarged in FIG. 3B. Table 2 showssimulation results on the absolute value |V_(DS)| (V) of thesource-drain voltage of the driving transistor 307 and the current valueI (nA) when the resistance of the light emitting element 308 is changedwith time from 10 MΩ to 12 MΩ to 15 MΩ in FIG. 3B.

Table 2

As shown in Table 2, when the resistance of the light emitting element308 is 10 MΩ, the same amount of current value, 500 nA flows in thelong-sized transistor and the standard-sized transistor. As theresistance of the light emitting element 308 rises to 12 MΩ with time,the current value of the long-sized transistor is reduced to 467 nAwhereas the current value of the standard-sized transistor is reduceddown to 419 nA. Compared to the initial current value (500 nA), the rateof change of the current value of the long-sized transistor is 93% andthe rate of change of the current value of the standard-sized transistoris 84%.

As time passes furthermore, the resistance of the light emitting element308 reaches 15 MΩ to reduce the current value of the long-sizedtransistor to 419 nA and the current value of the standard-sizedtransistor to 336 nA. At this point, the rate of change from the initialcurrent value (500 nA) is 84% for the long-sized transistor and 67% forthe standard-sized transistor.

The present invention is characterized in that a voltage is applied to agate electrode of the driving transistor and to a drain electrode orsource electrode thereof so that the driving transistor operates withits |V_(DS)| set equal to or larger than 1 V and equal to or smallerthan |V_(GS)−V_(th)|. In other words, the present invention ischaracterized in that a voltage is applied to a signal line forinputting a signal to the gate electrode of the driving transistor andis applied to a power supply line connected with the source electrode ordrain electrode of the driving transistor to give these linesappropriate electric potentials. According to the present invention, adriving transistor is operated at |V_(DS)| in the above range, therebymaking it possible to use the driving transistor as a resistor. Thismakes the value of the current flowing between electrodes of a lightemitting element inversely related to the sum of resistance of the lightemitting element and resistance of the driving transistor. As a result,the reduction with time in current value of the light emitting elementcan be slowed down. Accordingly, lowering of light emission luminancewith time is reduced and the reliability is improved.

Embodiment Mode 3

In the above embodiment modes, one transistor is used as a drivingtransistor. This embodiment gives with reference to FIG. 4D adescription on a case in which two transistors connected in series serveas a driving transistor.

A pixel 301 shown in FIG. 4D has a scanning line R_(j) and a drivingtransistor 321 in addition to the components of the pixel 301 of FIG.4A. To be specific, the pixel 301 of FIG. 4D has a switching transistor320, the driving transistor 321, a driving transistor 322, and a lightemitting element 308.

The description given in this embodiment deals with both simulationresults on a case of using one transistor for a driving transistor asshown in FIGS. 4B and 4C and simulation results on a case of using twotransistors for a driving transistor as shown in FIG. 4D.

Table 3 below shows the channel length L, the channel width W, theabsolute value |V_(th)| of the threshold, and the absolute value|V_(GS)| of the gate-source voltage for each transistor in thesimulation. As shown in Table 3, the sum of channel lengths of thetransistor 321 and the transistor 322 is 500 μm and is equal to thechannel length of the transistor 307 by itself. Every one of thesetransistors has the same channel width, 7 μm.

Table 3

Table 4 shows |V_(DS)| of each transistor and the current value I whenthe resistance of the light emitting element 308 is changed from 10 MΩto 15 MΩ with time. The simulation results shown in this embodiment modeare of when the resistance of the light emitting element 308 is changedfrom 10 MΩ to 15 MΩ with time with the border between the saturationrange and the non-saturation range as the starting point.

Table 4

As shown in Table 4, an increase in resistance of the light emittingelement 308 from 10 MΩ to 15 MΩ with time causes a change in currentvalue at a rate of 92% in the case where one transistor (the transistor307) is used. On the other hand, the rate of change in current value is98% when two transistors (the transistors 321 and 322) are used.

In short, the rate of change in current value with time is slowed downmore when the sum of channel lengths of plural transistors is 500 μmthan when one transistor having a channel length of 500 μm is used.

When plural transistors are used as a driving transistor, |V_(GS)| ofeach of the transistors is set arbitrarily. If each pixel has threesub-pixels, namely an R color sub-pixel, a G color sub-pixel, and a Bcolor sub-pixel, in order to display in multicolors, |V_(GS)| to beinputted is set to an arbitral value in accordance with the lightemission efficiency of the respective sub-pixels.

In the above Table 4, the rate of change when one transistor (thetransistor 307) is used as a driving transistor is 92%. On the otherhand, the rate of change when the long-sized transistor is employed is84% in Table 2. This is because, as can be understood from Tables 1 and2, and Tables 3 and 4, the rate of change varies depending on V_(GS) andV_(DS) even though L/W is the same and the initial current amount I_(D)is the same.

This embodiment mode can be combined freely with Embodiment Mode 1.

Embodiment Mode 4

In this embodiment mode, the configurations and operations of the signalline driving circuit 303, the scan line driving circuit 304, will bedescribed with reference to the FIG. 5, respectively.

First, the signal line driving circuit 303 is described with referenceto the FIG. 5A. The signal line driving circuit 303 has a shift register311, a first latch circuit 312 and a second latch circuit 313.

The operation of the signal driving circuit 303 is described briefly.The shift register 311 comprises a plurality of flip-flop Circuits (FF),and is supplied with a clock signal (S-CLK), a start pulse (S-SP), and aclock inversion signal (S-CLKb). Sampling pulses are output one by oneaccording to the timing of these signals.

The sampling pulse output from the shift register 311 is input into thefirst latch circuit 312. The first latch circuit 312 is supplied withdigital video signals, which, in turn, are retained in each columnaccording to the timing of the input of the sampling pulse.

In the first latch circuit 312, when the columns from the first to thelast are filled with the retained video signals, a latch pulse is inputinto the second latch circuit 313 during the horizontal return lineperiod. The video signals retained in the first latch circuit 312 aretransferred to the second latch circuit 313, at the same time. Then, theone line of the video signals retained in the second latch circuit 313is input into the signal lines S₁ to S_(n), at the same time.

While the video signals retained in the second latch circuit 313 arebeing input into the signal lines S₁ to S_(n), sampling pulses are againoutput from the shift register 311. The above operation is repeated.

Next, the scan line driving circuit 304 is described with reference tothe FIG. 5B. The scan line driving circuit 304 has a shift register 314and a buffer 315, respectively. Briefly, the shift register 314 outputssampling pulses one by one according to the clock signal (G-CLK), astart pulse (G-SP) and a clock inversion signal (G-CLKb). Next, thesampling pulses amplified in the buffer 315 are input into the scanline, and the scan line is turned to be a selected state one by one inresponse to the input of the sampling pulse. The pixel controlled by theselected scan line is supplied with digital video signals from signallines S₁ to S_(n) in sequence.

A level shifter circuit may be provided between the shift register 314and the buffer 315. By providing a level shifter circuit, the voltageamplitudes of the logic circuit part and the buffer can be altered.

This embodiment mode can be implemented in conjunction with embodimentmode 1 and/or 3.

Embodiment Mode 5

In this embodiment mode, a drive method applied to the present inventionwill be briefly described.

A drive method in the case where a multi-gradation image is displayed byusing a light emitting device, is broadly divided into an analoggradation method and a digital gradation method. Both methods can beapplied to the present invention. A differential point between both ofthe methods is a method of controlling a light emitting element inrespective states of light emission and non-light emission of the lightemitting element. The former analog gradation method is a method ofcontrolling the amount of current flowing into the light emittingelement to obtain gradation. The latter digital gradation method is amethod of driving the light emitting element with only two states of anon state (state in which luminance is substantially 100%) and an offstate (state in which luminance is substantially 0%).

With respect to the digital gradation method, a combination method of adigital gradation method and an area gradation method (hereinafterindicated as an area gradation method) and a combination method of adigital gradation method and a time gradation method (hereinafterindicated as a time gradation method) have been proposed in order torepresent a multi-gradation image.

The area gradation method is a method of dividing a pixel into aplurality of sub-pixels and selecting light emission or non-lightemission for the respective sub-pixels to represent gradation accordingto a difference between a light emitting area and the other area in apixel. In addition, the time gradation method is a method of controllinga period for which a light emitting element emits light to representgradation as reported in Patent Reference 2. Specifically, a frameperiod is divided into a plurality of sub-frame periods having differentlengths and light emission or non-light emission of the light emittingelement is selected for each of the periods to represent gradationaccording to a length of a light emitting period during the frameperiod.

Both the analog gradation method and the digital gradation method can beapplied to the light emitting device of the present invention. Further,both the area gradation method and the time gradation method areapplicable. Still further, other than the above methods, any knowndriving method can be applied to the light emitting device of thepresent invention.

On the other hand, when the digital gradation method is applied, all thepower source lines in the respective pixels may be set to the samepotential. Thus, the power source line can be commonly used betweenadjacent pixels.

Note that, in a light emitting device for conducting multi-colordisplay, a plurality of sub-pixels corresponding to respective colors ofR, G, and B are provided in a pixel. With respect to the respectivesub-pixels, because of a difference of current densities of respectivematerials for R, G, and B and a difference of transmittance of colorfilters therefor, there is the case where intensities of light emittedtherefrom are different even when the same voltage is applied.Therefore, when the potential of the power source line is changed foreach of sub-pixels corresponding to the respective colors, white balancewill be improved.

This embodiment mode can be arbitrarily combined with Embodiment modes 1to 4.

Embodiment Mode 6

The description given in this embodiment mode with reference to FIGS. 7Aand 7B is about an example of actual layout of the pixel 301 structuredas shown in FIG. 4B. FIG. 7A is a top view of the pixel 301 laid out andFIG. 7B is a sectional view taken along the line α-α′.

In FIG. 7A, reference symbol 306 denotes a switching transistor, and307, a driving transistor. Denoted by 5006 is a pixel electrode and 5007is a light emission area. In FIG. 7B, 5011 denotes a substrate, 5012 and5013, base films, 5014, a semiconductor, 5015, a gate insulating film,5016, a gate electrode, 5017, a first interlayer insulating film, 5018,a wire, 5019, a pixel electrode, 5020, a partition wall, and 5021, alight emitting layer.

In FIGS. 7A and 7B, the partition wall 5020 covers regions other thanthe light emission area 5007. The signal line S_(i) and the currentsupplying line V_(i) can be placed under the partition wall 5020. Thedriving transistor 307 can be placed under the source signal line S_(i)and the current supplying line V_(i).

By arranging the elements of the pixel in this way, the gate electrodeof the driving transistor overlaps a part of the power supply lineV_(i). Since the electric potential of the power supply line V_(i) isfixed, the capacitance between the gate electrode of the drivingtransistor 307 and the power supply line V_(i) can be used as a part ofthe capacitance for holding video signals.

In contrast to prior art which requires a capacitor element to holdV_(GS) of the driving transistor 307, V_(GS) in the present inventioncan be held sufficiently by the capacitance between the gate electrodeand channel formation region of the driving transistor 307. In short,the driving transistor 307 of the present invention can double as acapacitor and, in addition, fluctuation in characteristic of the drivingtransistor 307 itself can be reduced. Also, lowering of the apertureratio can be prevented by placing the driving transistor 307 under thepartition wall 5020.

The description given next with reference to FIGS. 8A and 8B is about anexample of actual layout of the pixel 301 structured as shown in FIG.4C. FIG. 8A is a top view of the pixel laid out and FIG. 8B is asectional view taken along the line α-α′.

In FIG. 8A, reference symbol 306 denotes a switching transistor, 307, adriving transistor, and 309, an erasing transistor. Denoted by 5608 is apixel electrode and 5609 is a light emission area. In FIG. 8B, 5611denotes a substrate, 5612 and 5613, base films, 5614, a semiconductor,5615, a gate insulating film, 5616, a gate electrode, 5617, a firstinterlayer insulating film, 5618, a wire, 5619, a pixel electrode, 5620,a partition wall, and 5621, a light emitting layer.

In the pixel having three transistors, the opening can be simplified bylining up two of the transistors: the switching transistor 306 and theerasing transistor 309 as shown in FIG. 8A. The driving transistor 307in FIG. 8A meanders in the longitudinal direction. This gives theopening a rectangular or similar shape and lowering of the apertureratio is thus avoided.

The shape of the driving transistor is not limited to those shown inFIGS. 7A and 8A. The driving transistor may have the letter U shape, theletter S shape, a spiral shape, or a meander shape.

The present invention employs a transistor with its L/W set to 10 orlarger and therefore is characterized in that V_(GS) of the drivingtransistor can be sufficiently held by a capacitor between the gateelectrode and channel formation region of the driving transistor. Inother words, a transistor in the present invention can double as acapacitor element and influence of fluctuation in characteristic of thetransistor itself is reduced.

Further, the present invention can be carried out by merely designing adriving transistor to have an L/W which is larger than usual and thereis no need to add another manufacturing step. Therefore the presentinvention can slow down the reduction of the current value withoutlowering the yield in the manufacturing process.

This embodiment mode can be combined arbitrarily with Embodiment Modes 1to 5.

Embodiment Mode 7

Electronic apparatuses to which the present invention is applied includea video camera, a digital camera, a goggles-type display (head mountdisplay), a navigation system, a sound reproduction apparatus (such as acar audio apparatus and an audio set), a lap-top computer, a gamemachine, a portable information terminal (such as a mobile computer, amobile telephone, a portable game machine, and an electronic book), animage reproduction apparatus including a recording medium (morespecifically, an apparatus which can reproduce a recording medium suchas a digital versatile disc (DVD) and so forth, and include a displayfor displaying the reproduced image), or the like. Specific examplesthereof are shown in FIG. 6.

FIG. 6A illustrates a light emitting device which includes a casing2001, a support table 2002, a display portion 2003, a speaker portion2004, a video input terminal 2005 and the like. The present invention isapplicable to the display portion 2003. The light emitting device is ofthe self-emission-type and therefore requires no backlight. Thus, thedisplay portion thereof can have a thickness thinner than that of theliquid crystal display device. The light emitting device is includingthe entire display device for displaying information, such as a personalcomputer, a receiver of TV broadcasting and an advertising display.

FIG. 6B illustrates a digital still camera which includes a main body2101, a display portion 2102, an image receiving portion 2103, anoperation key 2104, an external connection port 2105, a shutter 2106,and the like. The present invention can be applied to the displayportion 2102.

FIG. 6C illustrates a lap-top computer which includes a main body 2201,a casing 2202, a display portion 2203, a keyboard 2204, an externalconnection port 2205, a pointing mouse 2206, and the like. The presentinvention can be applied to the display portion 2203.

FIG. 6D illustrates a mobile computer which includes a main body 2301, adisplay portion 2302, a switch 2303, an operation key 2304, an infraredport 2305, and the like. The present invention can be applied to thedisplay portion 2302.

FIG. 6E illustrates a portable image reproduction apparatus including arecording medium (more specifically, a DVD reproduction apparatus),which includes a main body 2401, a casing 2402, a display portion A2403, another display portion B 2404, a recording medium (DVD or thelike) reading portion 2405, an operation key 2406, a speaker portion2407 and the like. The display portion A 2403 is used mainly fordisplaying image information, while the display portion B 2404 is usedmainly for displaying character information. The present invention canbe applied to these display portions A 2403 and B 2404. The imagereproduction apparatus including a recording medium further includes agame machine or the like.

FIG. 6F illustrates a goggle type display (head mounted display) whichincludes a main body 2501, a display portion 2502, arm portion 2503, andthe like. The present invention can be applied to the display portion2502.

FIG. 6G illustrates a video camera which includes a main body 2601, adisplay portion 2602, a casing 2603, an external connecting port 2604, aremote control receiving portion 2605, an image receiving portion 2606,a battery 2607, a sound input portion 2608, an operation key 2609, andthe like. The present invention can be applied to the display portion2602.

FIG. 6H illustrates a mobile telephone which includes a main body 2701,a casing 2702, a display portion 2703, a sound input portion 2704, asound output portion 2705, an operation key 2706, an external connectingport 2707, an antenna 2708, and the like. The present invention can beapplied to the display portion 2703. Note that the display portion 2703can reduce power consumption of the mobile telephone by displayingwhite-colored characters on a black-colored background.

When the brighter luminance of light emitted from the light emittingmaterial becomes available in the future, the light emitting device ofthe present invention will be applicable to a front-type or rear-typeprojector in which a light including output image information isenlarged by means of lenses or the like to be projected.

The aforementioned electronic apparatuses are more likely to be used fordisplay information distributed through a telecommunication path such asInternet, a CATV (cable television system), and in particular likely todisplay moving picture information. Since the response speed of thelight emitting materials is very high, the light emitting device ispreferably used for moving picture display.

A portion of the light emitting device that is emitting light consumespower, so it is desirable to display information in such a manner thatthe light-emitting portion therein becomes as small as possible.Accordingly, when the light emitting device is applied to a displayportion which mainly displays character information, e.g., a displayportion of a portable information terminal, and more particular, amobile telephone or a sound reproduction device, it is desirable todrive the light emitting device so that the character information isformed by a light emitting portion while a non-emission portioncorresponds to the background.

As set forth above, the present invention can be applied variously to awide range of electronic apparatuses in all fields. The electronicapparatuses in this embodiment mode can be obtained by utilizing a lightemitting device having a configuration in which the structures inEmbodiment modes 1 through 6 are freely combined.

The present invention is characterized in that a voltage is applied to agate electrode of the driving transistor and to a drain electrode orsource electrode thereof so that the driving transistor operates withits |V_(DS)| set equal to or larger than 1 V and equal to or smallerthan |V_(GS)−V_(th)|. In other words, the present invention ischaracterized in that a voltage is applied to a signal line forinputting a signal to the gate electrode of the driving transistor andis applied to a power supply line connected with the source electrode ordrain electrode of the driving transistor to give these linesappropriate electric potentials. Further, by operating a drivingtransistor at |V_(DS)| in the above range, it becomes possible to usethe driving transistor as a resistor. This makes the value of thecurrent flowing between electrodes of a light emitting element inverselyrelated to the sum of resistance of the light emitting element andresistance of the driving transistor. As a result, the reduction withtime in current value of the light emitting element can be slowed down.Accordingly, lowering of light emission luminance with time is reducedand the reliability is improved.

The present invention employs a transistor with its L/W set to 10 orlarger and therefore is characterized in that |V_(GS)| of the drivingtransistor is held by a capacitor between the gate electrode and channelformation region of the driving transistor. In other words, a transistorin the present invention can double as a capacitor element and influenceof fluctuation in characteristic of the transistor itself is reduced.

The present invention can be carried out by merely designing a drivingtransistor to have a channel length which is larger than usual and thereis no need to add another manufacturing step. Therefore the presentinvention can slow down the reduction of the current value withoutlowering the yield in the manufacturing process.

The resistance of the light emitting element is changed not only withtime but also by temperature shift because of its nature. To elaborate,the resistance of the light emitting element declines when thetemperature becomes higher than the normal temperature, namely roomtemperature, and rises when the temperature becomes lower than normal.Since a transistor of the present invention having L/W of 10 or more hasa gentle inclination of voltage-current characteristic, there is nolarge difference between V_(DS) and V_(EL), and the current value isgradually increased accompanying a change in V_(DS). In short, a changein resistance due to temperature shift does not cause a light emissionsurge, thereby preventing degradation of the light emitting element andburn-in of a display pattern.

TABLE 1 long-sized transistor standard-sized transistor L(μm) 500 5W(μm) 7 7 |V_(th)|(V) 2 2 |V_(GS)|(V) 9.7 5.1 101(V) 10 5.1 102(V) 0.3 0103(V) 5.023 5.014 104(V) 0 0 I(nA) 500 500 R(MΩ) 10 10

TABLE 2 long-sized transistor standard-sized transistor rate of rate ofchange of change of |VDS| current current (V) I (nA) value (%) |VDS| (V)I (nA) value (%) 10(MΩ) 4.997 500 0.086 500 12(MΩ) 4.387 467 93 0.071419 84 15(MΩ) 3.706 419 84 0.057 336 67

TABLE 3 transistor transistor transistor 307 321 322 L(μm) 500 250 250W(μm) 7 7 7 |V_(th)|(V) 2 2 2 |V_(GS)|(V) 9.1 7 7

TABLE 4 transistor 321 + transistor 307 transistor transistor transistor322 rate of change of 321 322 rate of change of |VDS| (V) I (nA) currentvalue (%) |VDS| (V) |VDS| (V) I (nA) current value (%) 10(MΩ) 7.1 495 55 503 15(MΩ) 5.2 460 92 3.1 4.5 497 98

What is claimed is:
 1. A method of driving a light emitting devicehaving a light emitting element and a driving transistor, the transistorbeing connected to the light emitting element, the transistor having achannel width W and a channel length L which satisfy L/W≧10, wherein avoltage is applied to a gate electrode of the driving transistor and toa drain electrode or source electrode thereof so that a gate-sourcevoltage V_(GS), source-drain voltage V_(DS), and threshold voltageV_(th) of the driving transistor satisfy 1≦|V_(DS)|≦V_(GS)−V_(th)|.